Why Do Interpretations Of Quantum Physics Matter?

A couple of weeks ago, fellow Forbes blogger Ethan Siegel took to his keyboard with the goal of making me sigh heavily, writing a post about interpretations of quantum physics calling the idea that you need an interpretation "the biggest myth in quantum physics." Ethan's argument boils down to noting that all of the viable interpretations known at present make identical predictions about the probability of getting particular outcomes for any experiment we might do. Therefore, according to Ethan, there's no need for any interpretation, because it doesn't really matter which of them you choose.

As an experimentalist by training and inclination, I am not without sympathy for this point of view. In fact, when I give talks about quantum mechanics and get the inevitable questions about interpretations, I tend to say something in that general vein-- that at present, nobody knows how to do an experiment that would distinguish between any of the viable interpretations. Given that, I say, the choice between interpretations is essentially an aesthetic one.

When, then, the heavy sigh on reading Ethan's post? There are two reasons why I had that reaction, and am writing this post in (belated) response.

The first reason is best explained via a historical analogy. For this, I would point to the infamous arguments between Albert Einstein and Niels Bohr that culminated in the famous paper by Einstein and his colleagues Boris Podolsky and Nathan Rosen that introduced the world to the physics of quantum entanglement. Einstein, Podolsky, and Rosen proposed a thought experiment involving a pair of particles prepared in such a way that their individual states were indeterminate but correlated-- most modern treatments make it a two-state system, so that the measurement of an individual particle has a 50/50 chance of coming up with either outcome, but when you make the same measurement on both particles, you're guaranteed to get the same answer.

This phenomenon turns out to be a rich source of interesting physics to explore, as you can tell from the fact that I've written at least ten posts about it (this one, and the nine links in the second paragraph). There's a thriving subfield of physics that has grown out of the ideas expressed in that one paper, generating both fascinating theoretical approaches and useful experimental technology.

That said, though, for the first three decades after it was published in 1935, the "EPR paper" was pretty much a footnote to physics. As late as the early 1980's, Abraham Pais's magisterial scientific biography of Einstein dispenses of it very quickly, as a sort of brief mis-step during the declining years of Einstein's scientific career.

Why was such an important paper so lightly regarded? Because it was, initially, an argument about interpretations of quantum physics. That is, Einstein, Podolsky, and Rosen used the correlation between separated measurements to argue that quantum mechanics must be incomplete-- that the measurement outcomes that orthodox quantum theory says are indeterminate are in fact determined in advance, and just hidden from us. They didn't disagree with the prediction of the quantum theory, only its interpretation.